Chimeric recombinant antigens of Toxoplasma gondii

ABSTRACT

The invention described herein relates to a method for combining antigen fragments of  Toxoplasma gondii  proteins, in the form of chimeric fusion products, and their use as diagnostic and immunogenic agents.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the national phase of International Application PCT/EP2006/001760 filed on Feb. 27, 2006 which, in turn, claims priority to European Application 05005065.7 filed on Mar. 8, 2005, the disclosures of both of which are incorporated herein by reference in their entirety. This application is also related to U.S. patent application Ser. No. 11/899,754 filed on Sep. 6, 2007, which is a continuation in part and claims priority to International Application PCT/EP2006/001760 filed on Feb. 27, 2006 which, in turn, claims priority to European Application 05005065.7 filed on Mar. 8, 2005.

FIELD OF THE INVENTION

The invention described herein relates to the technical field of the preparation of diagnostic means not applied directly to the animal or human body. The invention also furnishes compounds, methods for their preparation, methods for their use and compositions containing them, which are suitable for industrial application in the pharmaceutical and diagnostic field, particularly for the detection and diagnosis of Toxoplasma gondii infections, as well as for the treatment and prevention of said infections.

BACKGROUND OF THE INVENTION

Early diagnosis is a priority and highly desirable objective in all fields of medicament, particularly because it allows an appreciable improvement in the patient's life and a concomitant saving on the part of health care systems or on the part of the actual patients. In the particular case of the invention described herein, early diagnosis is that of potential or existing Toxoplasma gondii infection in pregnant women, with particular concern for the health of the foetus, and in infected subjects, particularly those with impaired immunity.

Toxoplasma gondii is an obligate intracellular parasite that infects all mammalian cells, including those of human subjects (McCabe and Remington, N. Engl. J. Med. 1988, 318-313-5). Morphologically, the parasite exhibits three distinct forms of infection: tachyzoite (asexual), bradyzoite (in tissue cysts, asexual) and sporozoite (in oocysts, sexual reproduction). Transmission typically occurs through ingestion of undercooked meat harbouring tissue cysts or vegetables contaminated with oocysts shed by cats. Human infection is generally asymptomatic and self-limiting in immunocompetent hosts. In contrast, in subjects with impaired immunity (particularly those affected by AIDS), toxoplasmosis is a severe opportunist infection, which may give rise to encephalitis with very serious outcomes (Luft, B. J., Remington J. S., 1992, Clin. Infect. Dis. 15, 211-22). Moreover, contracting primary infection during pregnancy may lead to miscarriages or to severe foetal disease in mammals.

For an extensive overview of the problem of toxoplasmosis the reader is referred to the specific medical literature.

Diagnosis of T. gondii infection is established by isolating the micro-organism in the blood or body fluids, identifying the parasite in tissues, detecting specific nucleotide sequences with PCR, or detecting specific anti-T. gondii immunoglobulins produced by the host in response to the infection (Beaman et al., 1995 Principles and Practice of Infectious Diseases 4th Ed., Churchill Livingstone Inc., New York, 2455-75; Remington J S et al. 1995, Infectious Diseases of the Fetus and Newborn Infant, W.B. Saunders, Philadelphia, Pa., 140-267).

Main challenges for clinicians are the diagnosis of primary T. gondii infections in pregnant women and the diagnosis of congenital infection in their newborns/infants. In both cases, to implement suitable therapies in good time and to avoid possible damage to the foetus and newborns/infants, it is very important to establish if the parasitic infection has been contracted before or after conception in pregnant women. Moreover, it is essential determining when the vertical transmission from the mother to the foetus occurred. Finally, for the clinical management of newborns/infants there is an urgent need of a sensitive diagnostic method than can discriminate, early in their life, between infected and uninfected subjects, both born to mothers with primary toxoplasmosis in pregnancy.

Seroconversion during gestation and diagnosis of congenital infection in neonates are generally done by attempting to detect the presence of the various classes of anti-Toxoplasma immunoglobulins (IgG, IgM, IgA, avidity of IgG), and to compare the immunological profiles of the mother versus her child. However, the available commercial assays do not provide enough sensitivity and specificity to allow a correct diagnosis of infection in all patients. Therefore the availability of specific, sensitive and innovative diagnostic agents is desirable.

T. gondii antigens have long been known and available, first of all as antigen mixtures obtained in various ways (FR 2,226,468, Mérieux; SU 533376, Veterinary Research Institute; JP 54044016, Nihon Toketsu Kanso), then as subsequent isolations of pure antigens (EP 0 082 745, Mérieux; EP 0 301 961, INSERM, Pasteur; WO 89/5658, Transgene) and their characterization both as proteins, and of their respective genes (WO 89/08700, U. Leland, Dartmouth Coll.; U.S. Pat. No. 4,877,726, Res. Inst. Palo Alto; WO 89/12683, INSERM, Pasteur; EP 0 391 319, Mochida Pharm.; IT 1,196,817, CNR; EP 0 431 541, Behringwerke; WO 92/01067, CNRS; WO 92/02624, U. Flinders; WO 92/11366, Innogenetics, Smithkline Beecham; U.S. Pat. No. 5,215,917, Res. Inst. Palo Alto; WO 92/25689, FR 2702491, INSERM, Pasteur; WO 96/02654, bioMeriéux, Transgene; EP 0 710 724 Akzo; EP 0 724 016, bioMeriéux; EP 0 751 147, Behringwerke; U.S. Pat. No. 5,633,139, Res. Inst. Palo Alto; WO 97/27300, Innogenetics; U.S. Pat. No. 5,665,542, U.S. Pat. No. 5,686,575, Res. Inst. Palo Alto; WO 99/32633, Heska; JP 11225783, Yano; WO 99/61906, Abbott; WO 99/66043, Smithkline Beecham; JP 2000300278, Yano; WO 00/164243, Virsol), and finally, the isolation and characterization of the antigenic regions of Toxoplasma gene products (WO 03/080839, Kenton S. r. l.)

Numerous studies have found various different antigenic proteins of T. gondii and the gene sequences of these have also been determined.

Among the most interesting proteins both for diagnostic and therapeutic purposes, in the form of vaccines, we should mention: the microneme proteins (WO 03/080839, Kenton S. r. l.; Beghetto et al., The Journal of Infectious Diseases, 2005, 191:637-645; Beghetto et al, International Journal for Parasitology, 2003, 33:163-173); the surface antigens SAG1 (or P30) (WO 89/08700, Stanford University; WO 89/12683 Pasteur, INSERM; WO 94/17813, WO 96/02654, Transgene, bioMeriéux; EP 0 724 016, WO 99/61906, U.S. Pat. No. 5,962,654, Harning et al., Clinical and Diagnostic Laboratory Immunology, May 1996, 355-357) and SAG2 (or P22) (Parmley et al., 1992, J. Clin. Microbiol. 30, 1127-33); the dense granule proteins GRA1 (or P24) (EP 0 301 961, Pasteur, INSERM; WO 89/05658, Transgene, Cesbron-Delauw, et al., 1989 P.N.A.S. USA 86, 7537-41), GRA2 (or P28) (WO 93/25689, INSERM, Pasteur; U.S. Pat. No. 5,633,139, U.S. Pat. No. 5,665,542, U.S. Pat. No. 5,686,575, Res. Inst. Palo Alto; Prince et al., Mol. Biochem. Parasitol., 34 3-14), GRA4 (Mevelec et al., Mol. Biochem. Parasitol 56, 227-38), GRA6 (or P32) (FR 2,702,491, INSERM, Pasteur; Lecordier al., Mol. Biochem. Parasitol. 70, 85-94), GRA7 (WO 99/61906, Abbott; Jacobs et al., Mol. Biochem. Parasitol. 91, 237-49) and GRA3 (Robben et al. 2002, J. Biol. Chem. 277, 17544-47): the rhoptry antigens ROP1 (or P66) (U.S. Pat. No. 5,976,553, U. Leland; EP 0 431 541, Innogenetics) and ROP2 (or P54) (Sharma et al., J. Immunol., 131, 377-83).

As described in the above-mentioned references, the antigens were obtained with recombinant cDNA techniques in expression vectors. For example, for the antigen SAG1, WO 98/08700 uses known expression vectors in phage lambda-gt11. WO 98/12683 uses the same phage and transfects E. coli with a proprietary plasmid, or by preparing a special expression cassette, as in WO 96/02654. EP 0 724 016 obtains mimotopes using combinatorial expression libraries of peptides. EP 0 301 961 describes how to obtain excretion-secretion antigens with molecular weights ranging from 20 kDa to 185 kDa. WO 89/05658 describes a protein containing the epitopes of the 24 kDa protein recognized by the antibodies produced against Toxoplasma excretion-secretion antigens; this protein is obtained by transfection of cells by means of expression vectors. WO 03/080839 describes a method based on phage-display technology for identifying antigen fragments of T. gondii proteins and their use as diagnostic and immunogenic agents. The antigen P28 (GRA2) is described in U.S. Pat. No. 5,633,139 and the method of obtaining it is again implemented through expression in phage lambda-gt11. The antigen P32 (GRA6) is described in patent FR 2,702,491, the antigen ROP1 (P66) in U.S. Pat. No. 5,976,553, P35 (or GRA8) in EP 0 431 541, WO 99/57295 and WO 99/61906, and lastly P68 in EP 0 431 541.

Yang et al. (Parasitol. Res., 2004, 92: 58-64) describe a chimeric protein containing SAG1 and SAG2 and its use to develop immunity against T. gondii in mice.

Chinese Patent 11 94991C discloses a recombinant fusion protein containing two toxoplasma antigens (GRA6 and p30). No data are reported to show that assays based on this recombinant fusion protein display the required sensitivity in IgG- and IgM-based tests.

During the last ten years, several studies have reported the use of recombinant antigens for the serological diagnosis of T. gondii infection. Nevertheless, although promising none of the assays based on recombinant antigens displayed all the characteristics required to replace the tachyzoite antigen in IgG- and IgM-based tests, indicating that further work is needed before an immunoassay employing recombinant products will be available for clinical purposes.

Thus the main aim of the studies in this filed is to improve the performance of enzyme-linked immunoassays based on recombinant products, thus improving, for example early diagnosis of congenital toxoplasmosis in newborns/infants.

SUMMARY OF THE INVENTION

It has now been found that the combination of antigenic regions of Toxoplasma gondii proteins, in the form of recombinant fusion products, retains the antigenic properties of the individual antigen fragments. The corresponding chimeric proteins thus produced can be used for diagnostic and therapeutic purposes.

The use of said chimeric antigens as diagnostic agents and the related diagnostic aids containing them, for example in the form of enzyme-linked immunoassays or kits, constitute a further object of the present invention.

Another object of the present invention are the gene sequences coding for the above-mentioned chimeric antigens, their use as medicaments, particularly for the prevention and therapy of Toxoplasma gondii infection, e.g. as gene therapy. The present invention also extends to the gene sequences that hybridize with the sequences of the above-mentioned chimeric antigens in stringent hybridization conditions.

Another object of the present invention is the use of the chimeric antigens as medicaments, particularly in the form of vaccines, which are useful for the prevention and cure of the infection. The vaccines according to the present invention are suitable for use in humans and other animals (particularly pig, cat, sheep).

These and other objects will be illustrated here below in detail, also by means of examples and figures.

DETAILED DESCRIPTION OF THE INVENTION

The main object of the present invention is, therefore, the provision of recombinant chimeric antigens obtained through the fusion of different antigenic regions of Toxoplasma gondii gene products, and the use of the recombinant proteins thus obtained for developing selective diagnostic and therapeutic means.

The main advantages of the present invention over the other types of antigens or antigen fragments known in the literature as reported above are the following and are evident when these antigens are used in diagnostic immunoassays on sample sera for detection of the infection:

-   -   With respect to the use of the entire Toxoplasma gondii antigen,         prepared from the parasite as lysed, whole-cell extract, the         chimeric recombinant antigens have the advantage of avoiding         unspecific reactions due to the presence of other         non-proteinaceous material and of providing a better         reproducibility. Moreover, some natural protein antigens of the         parasite are insoluble and, consequently, are poorly represented         in commercial assays employing the lysed, whole-cell extract         of T. gondii.     -   With respect to the use of single antigenic regions or single         antigen fragments (as described in WO 03/080839), the         recombinant chimeric antigens show the advantage of improving         the sensitivity of the assays in which they are used. In other         words their use decreases or abolishes the occurrence of false         negative responses.     -   A) With respect to the use of a mixture or a collection of         single antigenic regions (as also envisaged in WO 03/080839),         the advantages are least two. From the point of view of the         industrial applicability and production is much easier to         produce a single engineered construct containing three or more         antigen regions rather than separately produce each single         fragment and subsequently assemble them by an economic and         reproducible method. Secondly, as already said before, the use         of the chimeric recombinant antigens of the invention improves         the sensitivity of the assays.

These and other advantages are shown in the Examples section.

In particular the present invention relates to a chimeric recombinant antigen containing the fusion at least three different antigenic regions of Toxoplasma gondii, wherein said antigenic regions are B-cell epitopes, which bind to Toxoplasma gondii-specific antibodies. Preferably the Toxoplasma gondii-specific antibodies are extracted from sera of subjects who have been infected by Toxoplasma gondii.

More particularly the present invention covers a chimeric antigen, wherein the three different antigenic regions have an amino acid sequence selected from the group consisting of: SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, SEQ ID NO: 10, SEQ ID NO: 12, SEQ ID NO: 33, SEQ ID NO: 34, SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, SEQ ID NO: 40, SEQ ID NO: 41 and SEQ ID NO: 42. Preferred sequences in the above group are SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, SEQ ID NO: 10 and SEQ ID NO: 12.

For example the chimeric antigen of the invention comprises the amino acid sequence of SEQ ID NO: 28 or the amino acid sequence of SEQ ID NO: 30 or the amino acid sequence of SEQ ID NO: 32.

The chimeric antigens of the present invention may be engineered using known methods. The fusions may be direct (the C-terminus of one amino acid sequence is linked to the N-terminal of the other through a simple covalent bond) or they may employ a flexible linker domain, such as the hinge region of human IgG, or polypeptide linkers consisting of small amino acids such as glycine, serine, threonine or alanine, at various lengths and combinations. For example the linker may be a polyglycine repeat interrupted by serine or threonine at a certain interval. Preferably, the linker is composed by three glycine residues and two serine residues, giving the aminoacid sequence Ser-Gly-Gly-Gly-Ser (SGGGS linker) (SEQ ID 43).

Additionally, the chimeric antigens of the invention may be tagged by His-His-His-His-His-His (His6), to allow rapid purification by metal-chelate chromatography, and/or by epitopes to which antibodies are available, to allow for detection on western blots, immunoprecipitation, or activity depletion/blocking in bioassays.

Another object of the present invention is a nucleotide sequence coding for the chimeric antigen as defined above. According to a preferred embodiment of the invention such nucleotide sequence comprises at least three different nucleotide sequences selected from the group consisting of: SEQ ID NO: 1, SEQ ID NO: 3, SEQ ID NO: 5, SEQ ID NO: 7, SEQ ID NO: 9 and SEQ ID NO: 11. According to a more preferred embodiment, such nucleotide sequence comprises the nucleotide sequence of SEQ ID NO: 27 or the nucleotide sequence of SEQ ID NO: 29 or the nucleotide sequence of SEQ ID NO: 31.

Also included in the scope of the present invention are nucleotide sequences that hybridizes with any sequence described above under stringent hybridization conditions, as well the corresponding chimeric recombinant antigen encoded by such hybridized nucleotide sequence.

The chimeric antigens of the present invention may be prepared by cloning and expression in a prokaryotic or eukaryotic expression system, using the appropriate expression vectors. Any method known in the art can be employed.

For example the DNA molecules coding for the antigens of the invention are inserted into appropriately constructed expression vectors by techniques well known in the art (see Sambrook et al, 1989). Such vectors are another object of the present invention.

In order to be capable of expressing the desired protein (in this case the chimeric antigens), an expression vector should comprise also specific nucleotide sequences containing transcriptional and translational regulatory information linked to the DNA coding the desired protein in such a way as to permit gene expression and production of the protein. First in order for the gene to be transcribed, it must be preceded by a promoter recognizable by RNA polymerase, to which the polymerase binds and thus initiates the transcription process. There are a variety of such promoters in use, which work with different efficiencies (strong and weak promoters).

For eukaryotic hosts, different transcriptional and translational regulatory sequences may be employed, depending on the nature of the host. They may be derived form viral sources, such as adenovirus, bovine papilloma virus, Simian virus or the like, where the regulatory signals are associated with a particular gene, which has a high level of expression. Examples are the TK promoter of the Herpes virus, the SV40 early promoter, the yeast gal4 gene promoter, etc. Transcriptional initiation regulatory signals may be selected which allow for repression and activation, so that expression of the genes can be modulated. All these hosts are a further object of the present invention.

Nucleic acid molecules which encode the chimeric antigens of the invention may be ligated to a heterologous sequence so that the combined nucleic acid molecule encodes a fusion protein. Such combined nucleic acid molecules are included within the embodiments of the invention. For example, they may be joined to the DNA coding for a protein which allows purification of the chimeric antigen by only one step of affinity chromatography. This joined/fused protein may be for example Glutathione Sulpho Transferase (GST) to generate fusion products at the carboxy terminus of GST protein. The corresponding recombinant proteins expressed in the cytoplasm of transformed E. coli cells may be purified by affinity chromatography using a Glutathione-Sepharose resin.

The DNA molecule comprising the nucleotide sequence coding for the chimeric molecule of the invention is inserted into vector(s), having the operably linked transcriptional and translational regulatory signals, which is capable of integrating the desired gene sequences into the host cell. The cells which have been stably transformed by the introduced DNA can be selected by also introducing one or more markers which allow for selection of host cells which contain the expression vector. The marker may also provide for phototrophy to an auxotrophic host, biocide resistance, e.g. antibiotics, or heavy metals such as copper, or the like. The selectable marker gene can either be directly linked to the DNA gene sequences to be expressed, or introduced into the same cell by co-transfection. Additional elements may also be needed for optimal synthesis of proteins of the invention.

Factors of importance in selecting a particular plasmid or viral vector include: the ease with which recipient cells, that contain the vector may be recognized and selected form those recipient cells which do not contain the vector; the number of copies of the vector which are desired in a particular host; and whether it is desirable to be able to “shuttle” the vector between host cells of different species.

Once the vector(s) or DNA sequence containing the construct(s) has been prepared for expression the DNA construct(s) mat be introduced into an appropriate host cell by any of a variety of suitable means: transformation, transfection, conjugation, protoplast fusion, electroporation, calcium phosphate-precipitation, direct microinjection, etc.

Host cells may be either prokaryotic or eukaryotic. Example of eukaryotic hosts are mammalian cells, such as human, monkey, mouse, and Chinese hamster ovary (CHO) cells. Expression in these host cells provides post-translational modifications to protein molecules, including correct folding or glycosylation at correct sites. Also yeast cells can carry out post-translational peptide modifications including glycosylation. A number of recombinant DNA strategies exist which utilize strong promoter sequences and high copy number of plasmids which can be utilized for production of the desired proteins in yeast. Yeast recognizes leader sequences on cloned mammalian gene products and secretes peptides bearing leader sequences (i.e., pre-peptides). Example of prokaryotic hosts are bacteria, such as Escherichia coli.

After the introduction of the vector(s), the host cells are grown in a selective medium, which selects for the growth of vector-containing cells. Expression of the cloned gene sequence(s) results in the production of the desired proteins.

Purification of the recombinant antigens is carried out by any one of the methods known for this purpose, i.e. any conventional procedure involving extraction, precipitation, chromatography, electrophoresis, or the like. A further purification procedure that may be used in preference for purifying the antigens of the invention is affinity chromatography using monoclonal antibodies which bind the target protein and which are produced and immobilized on a gel matrix contained within a column. Impure preparations containing the recombinant protein are passed through the column. The antigens will be bound to the column by the specific antibody while the impurities will pass through. After washing, the antigen is eluted from the gel by a change in pH or ionic strength.

Another aspect of the present invention is the process for the recombinant production of the chimeric antigen as described above, comprising culturing the host cell transformed with the vector containing the nucleotide sequence of the invention and isolating the desired product.

A further object of the present invention is a DNA molecule comprising the DNA sequence coding for the above fusion protein, as well as nucleotide sequences substantially the same.

“Nucleotide sequences substantially the same” includes all other nucleic acid sequences which, by virtue of the degeneracy of the genetic code, also code for the given amino acid sequence.

Another object of the present invention is a nucleotide sequence which hybridizes to the complement of the nucleotide sequence coding for the chimeric antigen of the invention under highly stringent or moderately stringent conditions, as long as the antigen obtained maintains the same biological activity, i.e. ability to bind to antibodies against the parasite.

The term “hybridization” as used here refers to the association of two nucleic acid molecules with one another by hydrogen bonding. Typically, one molecule will be fixed to a solid support and the other will be free in solution. Then, the two molecules may be placed in contact with one another under conditions that favour hydrogen bonding. Factors that affect this bonding include: the type and volume of solvent; reaction temperature; time of hybridization; agitation; agents to block the non-specific attachment of the liquid phase molecule to the solid support (Denhardt's reagent or BLOTTO); the concentration of the molecules; use of compounds to increase the rate of association of molecules (dextran sulphate or polyethyleneglycol); and the stringency of the washing conditions following hybridization.

Stringency conditions are a function of the temperature used in the hybridization experiment, the molarity of the monovalent cations and the percentage of formamide in the hybridization solution. To determine the degree of stringency involved with any given set of conditions, one first uses the equation of Meinkoth et al. (1984) for determining the stability of hybrids of 100% identity expressed as melting temperature Tm of the DNA-DNA hybrid: Tm=81.5° C.+16.6 (LogM)+0.41 (% GC)−0.61 (% form)−500/L, where M is the molarity of monovalent cations, % GC is the percentage of G and C nucleotides in the DNA, % form is the percentage of formamide in the hybridization solution, and L is the length of the hybrid in base pairs. For each 1° C. that the Tm is reduced from that calculated for a 100% identity hybrid, the amount of mismatch permitted is increased by about 1%. Thus, if the Tm used for any given hybridization experiment at the specified salt and formamide concentrations is 10° C. below the Tm calculated for a 100% hybrid according to equation of Meinkoth, hybridization will occur even if there is up to about 10% mismatch.

As used herein, highly stringent conditions are those which are tolerant of up to about 15% sequence divergence, while moderately stringent conditions are those which are tolerant of up to about 20% sequence divergence. Without limitation, examples of highly stringent (12-15° C. below the calculated Tm of the hybrid) and moderately (15-20° C. below the calculated Tm of the hybrid) conditions use a wash solution of 2×SSC (standard saline citrate) and 0.5% SDS at the appropriate temperature below the calculated Tm of the hybrid. The ultimate stringency of the conditions is primarily due to the washing conditions, particularly if the hybridization conditions used are those which allow less stable hybrids to form along with stable hybrids. The wash conditions at higher stringency then remove the less stable hybrids. A common hybridization condition that can be used with the highly stringent to moderately stringent wash conditions described above is hybridization in a solution of 6×SSC (or 6×SSPE), 5×Denhardt's reagent, 0.5% SDS, 100 μg/ml denatured, fragmented salmon sperm DNA at a temperature approximately 20° C. to 25° C. below the Tm. If mixed probes are used, it is preferable to use tetramethyl ammonium chloride (TMAC) instead of SSC (Ausubel, 1987-1998).

The term “nucleic acid molecule” also includes analogues of DNA and RNA, such as those containing modified backbones.

The nucleic acid molecules of the invention also include antisense molecules that are partially complementary to nucleic acid molecules encoding antigens of the present invention and that therefore hybridize to the encoding nucleic acid molecules (hybridization). Such antisense molecules, such as oligonucleotides, can be designed to recognise, specifically bind to and prevent transcription of a target nucleic acid encoding a polypeptide of the invention, as will be known by those of ordinary skill in the art (see, for example, Cohen, J. S., Trends in Pharm. Sci., 10, 435 (1989), Okano, J. Neurochem. 56, 560 (1991); O'Connor, J. Neurochem 56, 560 (1991); Lee et al., Nucleic Acids Res 6, 3073 (1979); Cooney et al., Science 241, 456 (1988); Dervan et al., Science 251, 1360 (1991).

According to the terminology used herein, a composition containing a compound [X] is “substantially free of” impurities [herein, Y] when at least 85% by weight of the total X+Y in the composition is X. Preferably, X comprises at least about 90% by weight of the total of X+Y in the composition, more preferably at least about 95%, 98% or even 99% by weight.

Another aspect of the invention is the use of chimeric antigens described above as medicaments. In particular, one of the main objects of the invention is use of chimeric antigens as active ingredients for the preparation of medicaments for the prevention or treatment of Toxoplasma gondii infections.

In the case of gene therapy another object of the invention is the use of the nucleotide sequences coding for the antigens of the invention as medicaments, in particular for the preparation of medicaments useful for the treatment and prevention of Toxoplasma gondii infections.

The pharmaceutical compositions should preferably comprise a therapeutically effective amount of the chimeric antigens of the invention or the corresponding nucleotide sequence. Chimeric antigens of the invention may thus act as vaccines for the prevention or the treatment of Toxoplasma gondii infection.

For the therapeutic application, where the preparation of medicaments or vaccines comes within the framework of general knowledge for further reference the reader is again referred to the patent literature cited in the present description and, particularly, to Beghetto et al, The Journal of Infectious Diseases, 2005, 191:637-645.

The term “therapeutically effective amount” as used herein refers to an amount of a therapeutic agent needed to treat, ameliorate, or prevent a targeted disease or condition, or to exhibit a detectable therapeutic or preventative effect. For any compound, the therapeutically effective dose can be estimated initially either in cell culture assays, for example, of neoplastic cells, or in animal models, usually mice, rabbits, dogs, or pigs.

The animal model may also be used to determine the appropriate concentration range and route of administration. Such information can then be used to determine useful doses and routes for administration in humans.

The precise effective amount for a human subject will depend upon the severity of the disease state, general health of the subject, age, weight, and gender of the subject, diet, time and frequency of administration, drug combination (s), reaction sensitivities, and tolerance/response to therapy. This amount can be determined by routine experimentation and is within the judgement of the clinician. Generally, an effective dose will be from 0.01 mg/kg to 50 mg/kg, preferably 0.05 mg/kg to 10 mg/kg. Compositions may be administered individually to a patient or may be administered in combination with other agents, drugs or hormones.

A pharmaceutical composition may also contain a pharmaceutically acceptable carrier, for administration of a therapeutic agent. Such carriers include antibodies and other polypeptides, genes and other therapeutic agents such as liposomes, provided that the carrier does not itself induce the production of antibodies harmful to the individual receiving the composition, and which may be administered without undue toxicity.

Suitable carriers may be large, slowly metabolised macromolecules such as proteins, polysaccharides, polylactic acids, polyglycolic acids, polymeric amino acids, amino acid copolymers and inactive virus particles.

Pharmaceutically acceptable salts can be used therein, for example, mineral acid salts such as hydrochlorides, hydrobromides, phosphates, sulphates, and the like; and the salts of organic acids such as acetates, propionates, malonates, benzoates, and the like. A thorough discussion of pharmaceutically acceptable carriers is available in Remington's Pharmaceutical Sciences (Mack Pub. Co., N.J. 1991).

Pharmaceutically acceptable carriers in therapeutic compositions may additionally contain liquids such as water, saline, glycerol and ethanol. Additionally, auxiliary substances, such as wetting or emulsifying agents, pH buffering substances, and the like, may be present in such compositions. Such carriers enable the pharmaceutical compositions to be formulated as tablets, pills, dragees, capsules, liquids, gels, syrups, slurries, suspensions, and the like, for ingestion by the patient.

Once formulated, the compositions of the invention can be administered directly to the subject. The subjects to be treated can be animals; in particular, human subjects can be treated.

The pharmaceutical compositions utilised in this invention may be administered by any number of routes including, but not limited to, oral, intravenous, intramuscular, intra-arterial, intramedullary, intrathecal, intraventricular, transdermal or transcutaneous applications (for example, see W098/20734), subcutaneous, intraperitoneal, intranasal, enteral, topical, sublingual, intravaginal or rectal means. Gene guns or hyposprays may also be used to administer the pharmaceutical compositions of the invention. Typically, the therapeutic compositions may be prepared as injectables, either as liquid solutions or suspensions; solid forms suitable for solution in, or suspension in, liquid vehicles prior to injection may also be prepared.

Direct delivery of the compositions will generally be accomplished by injection, subcutaneously, intraperitoneally, intravenously or intramuscularly, or delivered to the interstitial space of a tissue. The compositions can also be administered into a lesion.

Dosage treatment may be a single dose schedule or a multiple dose schedule.

The method of treating a mammal suffering from Toxoplasma-gondii infection, comprising administering a therapeutically effective amount of the vaccine as described above represents one of the aspects of the present invention.

A further object of the present invention is the use of chimeric antigens as described above as active agents for the diagnosis of Toxoplasma gondii infections, in particular for the diagnosis of the time of infection.

Also the kits for the diagnosis of Toxoplasma gondii infection, containing at least one chimeric antigen according are part of the present invention. Such kits may be useful for the diagnosis of an acute and/or chronic Toxoplasma gondii infection.

The chimeric antigen of the invention may be employed in virtually any assay format that employs a known antigen to detect antibodies. A common feature of all of these assays is that the antigen is contacted with the body component suspected of containing antibodies under conditions that permit the antigen to bind to any such antibody present in the component. Such conditions will typically be physiologic temperature, pH and ionic strength using an excess of antigen. The incubation of the antigen with the specimen is followed by detection of immune complexes comprised of the antigen.

Design of the immunoassays is subject to a great deal of variation, and many formats are known in the art. Protocols may, for example, use solid supports, or immunoprecipitation. Most assays involve the use of labeled antibody or polypeptide; the labels may be, for example, enzymatic, fluorescent, chemiluminescent, radioactive, or dye molecules. Assays which amplify the signals from the immune complex are also known; examples of which are assays which utilize biotin and avidin, and enzyme-labeled and mediated immunoassays, such as ELISA assays.

The immunoassay may be, without limitation, in a heterogenous or in a homogeneous format, and of a standard or competitive type. In a heterogeneous format, the polypeptide is typically bound to a solid matrix or support to facilitate separation of the sample from the polypeptide after incubation.

Examples of solid supports that can be used are nitrocellulose (e.g., in membrane or microtiter well form), polyvinyl chloride (e.g., in sheets or microtiter wells), polystyrene latex (e.g., in beads or microtiter plates, polyvinylidine fluoride (known as Immulon™), diazotized paper, nylon membranes, activated beads, and Protein A beads. For example, Dynatech Immulon™1 or Immulon™2 microtiter plates or 0.25 inch polystyrene beads (Precision Plastic Ball) can be used in the heterogeneous format. The solid support containing the antigenic polypeptides is typically washed after separating it from the test sample, and prior to detection of bound antibodies. Both standard and competitive formats are known in the art.

In a homogeneous format, the test sample is incubated with the combination of antigens in solution. For example, it may be under conditions that will precipitate any antigen-antibody complexes which are formed. Both standard and competitive formats for these assays are known in the art.

In a standard format, the amount of antibodies forming the antibody-antigen complex is directly monitored. This may be accomplished by determining whether labeled anti-xenogenic (e.g., anti-human) antibodies which recognize an epitope on anti-Toxoplasma gondii antibodies will bind due to complex formation. In a competitive format, the amount of antibodies in the sample is deduced by monitoring the competitive effect on the binding of a known amount of labeled antibody (or other competing ligand) in the complex.

Complexes formed comprising anti-Toxoplasma gondii antibody (or, in the case of competitive assays, the amount of competing antibody) are detected by any of a number of known techniques, depending on the format. For example, unlabeled antibodies in the complex may be detected using a conjugate of antixenogeneic Ig complexed with a label, (e.g., an enzyme label).

In an immunoprecipitation or agglutination assay format the reaction between the chimeric antigens and the antibody forms a network that precipitates from the solution or suspension and forms a visible layer or film of precipitate. If no anti-Toxoplasma gondii antibody is present in the test specimen, no visible precipitate is formed.

The chimeric antigens of the invention will typically be packaged in the form of a kit for use in these immunoassays. The kit will normally contain in separate containers the combination of antigens (either already bound to a solid matrix or separate with reagents for binding them to the matrix), control antibody formulations (positive and/or negative), labeled antibody when the assay format requires same and signal generating reagents (e.g., enzyme substrate) if the label does not generate a signal directly. Instructions (e.g., written, tape, VCR, CD-ROM, etc.) for carrying out the assay usually will be included in the kit.

The diagnostic kits, which are the object of the present invention, are therefore known to the expert in the field. By way of an example, the reader is referred to the patent literature cited above, to which may be added U.S. Pat. No. 6,265,176, WO 01/63283, and WO 03/080839 as further references.

The invention will now be illustrated in greater detail by means of examples and figures.

DESCRIPTION OF THE FIGURES

FIG. 1. Plasmid map of the bacterial expression vector pGEX-SN.

FIG. 2. Schematic representation of the chimeric antigens.

The DNA sequences of clones Tx-2.a, Tx-1.16, Tx-4.18, Tx-15.11, Tx-1.11 and Tx-11.b, respectively encoding for protein fragments of the T. gondii genes MIC2, MIC3, SAG1, GRA3, GRA7 and M2AP were used for the construction of GST-EC2, GST-EC3 and GST-EC4 fusion proteins.

FIG. 3. Expression of T. gondii chimeric antigens in E. coli cells.

SDS-PAGE analysis of purified GST-EC2, GST-EC3 and GST-EC4 fusion proteins. The recombinant proteins were subjected to electrophoresis (0.003 mg/lane) on 12% acrylamide gel. MW, molecular weight markers.

FIG. 4. Antigenic properties of individual protein fragments within the chimeric antigens.

Immunoreactivity of individual Tx-2.a, Tx-1.16, Tx-4.18, Tx-15.11, Tx-1.11 and Tx-11.b antigen fragments, and of EC2, EC3 and EC4 chimeric antigens with serum samples from T. gondii infected individuals. Sera were used either as whole speciments (serum) or after depletion of specific antibodies against combinations of antigen fragments (Tx-1.16/Tx-4.18 depletion, Tx-2.a/Tx-4.18 depletion, etc.).

EXAMPLES

The following Table 1 gives, by way of examples, the DNA sequences used for the construction of recombinant Toxoplasma gondii chimeric antigens:

TABLE 1 Name Sequence Identification Classification Tx-15.11 GCTGCCTTGGGAGGCCTTGCGGCGGATC GRA 3 Dense (SEQ ID 1) AGCCTGAAAATCATCAGGCTCTTGCAGAA granule CCAGTTACGGGTGTGGGGGAAGCAGGA protein GTGTCCCCCGTCAACGAAGCTGGTGAGT CATACAGTTCTGCAACTTCGGGTGTCCAA GAAGCTACCGCCCCAGGTGCAGTGCTCC TGGACGCAATCGATGCCGAGTCGGATAA GGTGGACAATCAGGCGGAGGGAGGTGA GCGTATGAAGAAGGTCGAAGAGGAGTTG TCGTTATTGAGGCGGGAATTATATGATCG CACAGATCGCCCTGGT Tx-1.11 CAGTTCGCTACCGCGGCCACCGCGTCAG GRA 7 Dense (SEQ ID 3) ATGACGAACTGATGAGTCGAATCCGAAAT granule TCTGACTTTTTCGATGGTCAAGCACCCGT protein TGACAGTCTCAGACCGACGAACGCCGGT GTCGACTCGAAAGGGACCGACGATCACC TCACCACCAGCATGGATAAGGCATCTGTA GAGAGTCAGCTTCCGAGAAGAGAGCCAT TGGAGACGGAGCCAGATGAACAAGAAGA AGTTCAT Tx-1.16 AGGAGGACTGGATGTCATGCCTTCAGGG MIC 3 Microneme (SEQ ID 5) AGAACTGCAGCCCTGGTAGATGTATTGAT protein GACGCCTCGCATGAGAATGGCTACACCT GCGAGTGCCCCACAGGGTACTCACGTGA GGTGACTTCCAAGGCGGAGGAGTCGTGT GTGGAAGGAGTCGAAGTCACGCTGGCTG AGAAATGCGAGAAGGAATTCGGCATCAG CGCGTCATCCTGCAAATGCGAT Tx-4.18 CCATCGGTCGTCAATAATGTCGCAAGGT SAG 1 Surface (SEQ ID 7) GCTCCTACGGTGCAGACAGCACTCTTGG protein TCCTGTCAAGTTGTCTGCGGAAGGACCC ACTACAATGACCCTCGTGTGCGGGAAAG ATGGAGTCAAAGTTCCTCAAGACAACAAT CAGTACTGTTCCGGGACGACGCTGACTG GTTGCAACGAGAAATCGTTCAAAGATATT TTGCCAAAATTAACTGAGAACCCGTGGCA GGGTAACGCTTCGAGTGATAAGGGTGCC ACGCTAACGATCAAGAAGGAAGCATTTCC AGCCGAGTCAAAAAGCGTCATTATTGGAT GCACAGGGGGATCGCCTGAGAAGCATCA CTGTACCGTGAAACTGGAGTTTGCCGGG GCTGCAGGGTCAGCAAAATCGGCT Tx-2.a CCCCAGGATGCCATTTGCTCGGATTGGT MIC2 Microneme (SEQ ID 9) CCGCATGGAGCCCCTGCAGTGTATCCTG protein CGGTGACGGAAGCCAAATCAGGACGCGA ACTGAGGTTTCTGCTCCGCAACCTGGAA CACCAACATGTCCGGACTGCCCTGCGCC CATGGGAAGGACTTGCGTGGAACAAGGC GGACTTGAAGAAATCCGTGAATGCAGTG CGGGGGTATGTGCTGTTGACGCTGGATG TGGCGTCTGGGTT Tx-11.b AACGAACCGGTGGCCCTAGCTCAGCTCA M2AP Microneme (SEQ ID 11) GCACATTCCTCGAGCTCGTCGAGGTGCC protein ATGTAACTCTGTTCATGTTCAGGGGGTGA TGACCCCGAATCAAATGGTCAAAGTGACT GGTGCAGGATGGGATAATGGCGTTCTCG AGTTCTATGTCACGAGGCCAACGAAGAC AGGCGGGGACACAAGCCGAAGCCATCTT GCGTCGATCATGTGTTATTCCAAGGACAT TGACGGCGTGCCGTCAGACAAAGCGGGA AAGTGCTTTCTGAAGAACTTTTCTGGTGA AGACTCGTCGGAAATAGACGAAAAAGAA GTATCTCTACCCATCAAGAGCCACAACGA TGCGTTCATGTTCGTTTGTTCTTCAAATGA TGGATCCGCACTCCAGTGTGATGTTTTCG CCCTTGATAACACCAACTCTAGCGACGG GTGGAAAGTGAATACCGTGGATCTTGGC GTCAGCGTTAGTCCGGATTTGGCATTCG GACTCACTGCAGATGGGGTCAAGGTGAA GAAGTTGTACGCAAGCAGCGGCCTGACA GCGATCAACGACGACCCTTCCTTGGGGT GCAAGGCTCCTCCCCATTCTCCGCCGGC CGGAGAGGAACCGAGTTTGCCGTCGCCT GAAAACAGCGGGTCTGCAACACCAGCGG AAGAAAGTCCGTCTGAGTCTGAATCT

The sequence Tx-15.11 constitutes a fragment of the gene GRA3 (Bermudes et al., Mol. Biochem. Parasitol., 1994, 68:247-257). Said clone has the amino acid sequence AALGGLAADQPENHQALAEPVTGVGEAGVSPVNEAGESYSSATSGVQEATA PGAVLLDAIDAESDKVDNQAEGGERMKKVEEELSLLRRELYDRTDRPG (SEQ ID 2) and its use in chimeric antigens is covered by the present invention.

The sequence Tx-1.11 constitutes a fragment of the antigen GRA7 (Bonhomme et al., J. Histochem. Cytochem., 1998, 46:1411-1421). Said clone has the amino acid sequence ATAATASDDELMSRIRNSDFFDGQAPVDSLRPTNAGVDSKGTDDHLTTSMDK ASVESQLPRREPLETEPDEQEEVHF (SEQ ID 4) and its use in chimeric antigens is covered by the present invention.

The sequence Tx-1.16 constitutes a fragment of the MIC3 gene (Garcia-Réguet et al., Cellular Microbiol., 2000, 2:353-364). Said clone has the amino acid sequence RRTGCHAFRENCSPGRCIDDASHENGYTCECPTGYSREVTSKAEESCVEGV EVTLAEKCEKEFGISASSCKCD (SEQ ID 6) and its use in chimeric antigens is covered by the present invention.

The sequence Tx-4.18 constitutes a fragment of the antigen SAG1 (Burg et al., J. Immunol., 1988, 141:3584-3591). Said clone has the amino acid sequence PSVVNNVARCSYGADSTLGPVKLSAEGPTTMTLVCGKDGVKVPQDNNQYCS GTTLTGCNEKSFKDILPKLTENPWQGNASSDKGATLTIKKEAFPAESKSVIIGC TGGSPEKHHCTVKLEFAGAAGSAKSA (SEQ ID 8) and its use in chimeric antigens is covered by the present invention.

The sequence Tx-2.a represents a fragment of the MIC2 gene (Wan et al, Mol. Biochem. Parasitol., 1997, 84:203-214). Said clone has the amino acid sequence PQDAICSDWSAWSPCSVSCGDGSQIRTRTEVSAPQPGTPTCPDCPAPMGRT CVEQGGLEEIRECSAGVCAVDAGCGVWV (SEQ ID 10) and its use in chimeric antigens is covered by the present invention.

The sequence Tx-11.b represents a distinct fragment of the M2AP gene (Rabenau et al., Mol. Microbiol., 2001, 41:537-547). Said clone has the amino acid sequence NEPVALAQLSTFLELVEVPCNSVHVQGVMTPNQMVKVTGAGWDNGVLEFYV TRPTKTGGDTSRSHLASIMCYSKDIDGVPSDKAGKCFLKNFSGEDSSEIDEKE VSLPIKSHNDAFMFVCSSNDGSALQCDVFALDNTNSSDGWKVNTVDLGVSVS PDLAFGLTADGVKVKKLYASSGLTAINDDPSLGCKAPPHSPPAGEEPSLPSPE NSGSATPAEESPSESES (SEQ ID 12) and its use in chimeric antigens is covered by the present invention.

Construction of Chimeric Antigens

EC2 protein product is a chimeric molecule containing the DNA sequences Tx-2.a, Tx-1.16 and Tx-4.18. SEQ ID 9 was used as template for DNA amplification of clone Tx-2.a by using oligonucleotides K551 (5′-GGACTAGTCGGCTCCCCCAGGATGCC-3′) (SEQ ID 13) and K553 (5′-CATCCAGTCCTGCTACCGCCACCAGACCAGACGCCACATCC AGC-3′) (SEQ ID 14). The oligonucleotide K553 contains a sequence encoding for the linker SGGGS (SEQ ID 43), which joins the sequences Tx-2.a and Tx-1.16. PCR condition was 30″ at 94° C., 30″ at 50° C. and 60″ at 72° C. for 20 cycles.

SEQ ID 5 was used as template for DNA of clone Tx-1.16 by using oligonucleotides K552 (5′-GTGGCGTCTGGTCTGGTGGCGGTAGCAG GACTGGATGTCATGCC-3′) (SEQ ID 15) and K555 (5′-TGACGACCGAGCTAC CGCCACCAGAGTTATCGCATTTGCAGGATG-3′) (SEQ ID 16). The oligonucleotide K555 contains a sequence encoding for the linker SGGGS (SEQ ID 43), which joins the sequences Tx-1.16 and Tx-4.18. PCR condition was 30″ at 94° C., 30″ at 50° C. and 60″ at 72° C. for 20 cycles.

SEQ ID 7 was used as template for DNA amplification of clone Tx-4.18 by using oligonucleotides K554 (5′-ATGCGATAACTCTGGTGGCGG TAGCTCGGTCGTCAATAATGTCGC-3′) (SEQ ID 17) and K556 (5′-CCGCGGCC GCTAGCCGATTTTGCTGACCCTG-3′) (SEQ ID 18) PCR condition was 30″ at 94° C., 30″ at 50° C. and 60″ at 72° C. for 20 cycles.

The PCR products were purified by means of the “Qiagen Purification Kit” (Qiagen, CA, USA). 25 ng of DNA amplification products of SEQ ID 9 and SEQ ID 5 were mixed together and used as templates in PCR reaction by using oligonucleotides K551 and K555. PCR condition was 30″ at 94° C., 30″ at 50° C. and 60″ at 72° C. for 20 cycles. 25 ng of the resulting DNA amplification was purified with “Qiagen Purification Kit” (Qiagen, CA, USA) and then mixed with 25 ng of DNA amplification product of SEQ ID 4. Finally, the DNA mixture was used as template for DNA amplification by using oligonucleotides K551 and K556, following PCR condition of 30″ at 94° C., 30″ at 50° C. and 90″ at 72° C. for 20 cycles.

EC3 protein product is a chimeric molecule containing the DNA sequences Tx-15.11, Tx-1.11 and Tx-11.b.

SEQ ID 1 was used as template for DNA amplification of clone Tx-15.11 by using oligonucleotides K563 (5′-GGACTAGTCGGCTGG CTGCCTTGGGAGGCCTTG-3′) (SEQ ID 19) and K565 (5′-GCCGCGGTAGCACTACCG CCACCAGACAAACCAGGGCGATCTGTG-3′) (SEQ ID 20). The oligonucleotide K565 contains a sequence encoding for the linker SGGGS (SEQ ID 43), which joins the sequences Tx-15.11 and Tx-1.11. The PCR protocol was 30″ at 94° C., 30″ at 48° C. and 60″ at 72° C. for 20 cycles.

SEQ ID 3 was used as template for DNA amplification of clone Tx-1.11 by using oligonucleotides K564 (5′-GCCCTGGTTTGTCTGGTGGCGGTAG TGCTACCGCGGCCACCGCG-3′) (SEQ ID 21) and K567 (5′-CCGGTTCGTTACTACCG CCACCAGAGAAATGAACTTCTTCTTGTTC-3′) (SEQ ID 22). The oligonucleotide K567 contains a sequence encoding for the linker SGGGS (SEQ ID 43), which joins the sequences Tx-1.11 and Tx-11.b. The PCR protocol was 30″ at 94° C., 30″ at 48° C. and 60″ at 72° C. for 20 cycles.

SEQ ID 11 was used as template for DNA amplification of clone Tx-1.11 by using oligonucleotides K564 (5′-GCCCTGGTTTGTCTGGTGGCGGTAG TGCTACCGCGGCCACCGCG-3′) (SEQ ID 23) and K567 (5′-CCGGTTCGTTACTACCG CCACCAGAGAAATGAACTTCTTCTTGTTC-3′) (SEQ ID 24). The oligonucleotide K567 contains a sequence encoding for the linker SGGGS (SEQ ID 43), which joins the sequences Tx-1.11 and Tx-11.b. The PCR protocol was 30″ at 94° C., 30″ at 48° C. and 60″ at 72° C. for 20 cycles.

The PCR products were purified by means of the “Qiagen Purification Kit” (Qiagen, CA, USA). 25 ng of DNA amplification products of SEQ ID 1 and SEQ ID 3 were mixed together and used as templates in PCR reaction by using oligonucleotides K563 and K567. The PCR protocol was 30″ at 94° C., 30″ at 45° C. and 60″ at 72° C. for 30 cycles. 25 ng of the resulting DNA amplification was purified and then mixed with 25 ng of DNA amplification product of SEQ ID 11. Finally, the DNA mixture was used as template for DNA amplification by using oligonucleotides K563 and K568, following PCR condition of 30″ at 94° C., 30″ at 45° C. and 180″ at 72° C. for 30 cycles.

EC4 protein product is a chimeric molecule containing the DNA sequences Tx-2.a, Tx-1.16 and Tx-11.b.

SEQ ID 9 was used as template for DNA amplification of clone Tx-2.a using oligonucleotides K551 and K553.

SEQ ID 5 was used as template for DNA amplification of clone Tx-1.16 by using oligonucleotides K552 and K572 (5′-CGTTACTACCGC CACCAGAGTTATCGCATTTGCAGGATGA-3′) (SEQ ID 25). The oligonucleotide K572 contains a sequence encoding for the linker SGGGS (SEQ ID 43), which joins the sequences Tx-1.16 and Tx-11.b.

SEQ ID 11 was used as template for DNA amplification of clone Tx-11.b by using oligonucleotides K571 (5′-TAACTCTGGTGGCGGTAGT AACGAACCGGTGGCCCTAGC-3′) (SEQ ID 26) and K568.

The PCR products were purified as by using “Qiagen Purification Kit” (Qiagen, CA, USA). 25 ng of DNA amplification products of SEQ ID 9 and SEQ ID 5 were mixed together and used as templates in PCR reaction by using oligonucleotides K551 and K572. 25 ng of the resulting DNA amplification was purified and then mixed with 25 ng of DNA amplification product of SEQ ID 11. Finally, the DNA mixture was used as template for DNA amplification by using oligonucleotides K551 and K568. PCR conditions for the construction of EC4 were the same that those used for EC2 and EC3 constructs.

The following Table 2 gives, by way of examples, the DNA sequences of the EC2, EC3 and EC4 chimeric antigens:

TABLE 2 Name Sequence EC2 ACTAGTCGGCTCCCCCAGGATGCCATTTGCTCGGATTGGTC SEQ CGCATGGAGCCCCTGCAGTGTATCCTGCGGTGACGGAAGC ID 27 CAAATCAGGACGCGAACTGAGGTTTCTGCTCCGCAACCTGG AACACCAACATGTCCGGACTGCCCTGCGCCCATGGGAAGGA CTTGCGTGGAACAAGGCGGACTTGAAGAAATCCGTGAATGC AGTGCGGGGGTATGTGCTGTTGACGCTGGATGTGGCGTCT GGTCTGGTGGCGGTAGCAGGACTGGATGTCATGCCTTCAG GGAGAACTGCAGCCCTGGTAGATGTATTGATGACGCCTCGC ATGAGAATGGCTACACCTGCGAGTGCCCCACAGGGTACTCA CGTGAGGTGACTTCCAAGGCGGAGGAGTCGTGTGTGGAAG GAGTCGAAGTCACGCTGGCTGAGAAATGCGAGAAGGAATTC GGCATCAGCGCGTCATCCTGCAAATGCGATAACTCTGGTGG CGGTAGCTCGGTCGTCAATAATGTCGCAAGGTGCTCCTACG GTGCAGACAGCACTCTTGGTCCTGTCAAGTTGTCTGCGGAA GGACCCACTACAATGACCCTCGTGTGCGGGAAAGATGGAGT CAAAGTTCCTCAAGACAACAATCAGTACTGTTCCGGGACGA CGCTGACTGGTTGCAACGAGAAATCGTTCAAAGATATTTTGC CAAAATTAACTGAGAACCCGTGGCAGGGTAACGCTTCGAGT GATAAGGGTGCCACGCTAACGATCAAGAAGGAAGCATTTCC AGCCGAGTGAAAAAGCGTCATTATTGGATGCACAGGGGGAT CGCCTGAGAAGCATCACTGTACCGTGAAACTGGAGTTTGCC GGGGCTGCAGGGTCAGCAAAATCGGCTAGCGGCCGC EC3 ACTAGTCGGCTGGCTGCCTTGGGAGGCCTTGCGGATCAGC SEQ CTGAAAATCATCAGGCTCTTGCAGAACCAGTTACGGGTGTG ID 29 GGGGAAGCAGGAGTGTCCCCCGTCAACGAAGCTGGTGAGT CATACAGTTCTGCAACTTCGGGTGTCCAAGAAGCTACCGCC CCAGGTGCAGTGCTCCTGGACGCAATCGATGCCGAGTCGG ATAAGGTGGACAATCAGGCGGAGGGAGGTGAGCGTATGAA GAAGGTCGAAGAGGAGTTGTCGTTATTGAGGCGGGAATTAT ATGATCGCACAGATCGCCCTGGTTTGTCTGGTGGCGGTAGT GCTACCGCGGCCACCGCGTCAGATGACGAACTGATGAGTC GAATCCGAAATTCTGACTTTTTCGATGGTCAAGCACCCGTTG ACAGTCTCAGACCGACGAACGCCGGTGTCGACTCGAAAGG GACCGACGATCACCTCACCACCAGCATGGATAAGGCATCTG TAGAGAGTCAGCTTCCGAGAAGAGAGCCATTGGAGACGGAG CCAGATGAACAAGAAGAAGTTCATTTCTCTGGTGGCGGTAG TAACGAACCGGTGGCCCTAGCTCAGCTCAGCACATTCCTCG AGCTCGTCGAGGTGCCATGTAACTCTGTTCATGTTCAGGGG GTGATGACCCCGAATCAAATGGTCAAAGTGACTGGTGCAGG ATGGGATAATGGCGTTCTCGAGTTCTATGTCACGAGGCCAA CGAAGACAGGCGGGGACACAAGCCGAAGCCATCTTGCGTC GATCATGTGTTATTCCAAGGACATTGACGGCGTGCCGTCAG ACAAAGCGGGAAAGTGCTTTCTGAAGAACTTTTCTGGTGAAG ACTCGTCGGAAATAGACGAAAAAGAAGTATCTCTACCCATCA AGAGCCACAACGATGCGTTCATGTTCGTTTGTTCTTCAAATG ATGGATCCGCACTCCAGTGTGATGTTTTCGCCCTTGATAACA CCAACTCTAGCGACGGGTGGAAAGTGAATACCGTGGATCTT GGCGTCAGCGTTAGTCCGGATTTGGCATTCGGACTCACTGC AGATGGGGTCAAGGTGAAGAAGTTGTACGCAAGCAGCGGC CTGACAGCGATCAACGACGACCCTTCCTTGGGGTGCAAGGC TCCTCCCCATTCTCCGCCGGCCGGAGAGGAACCGAGTTTGC CGTCGCCTGAAAACAGCGGGTCTGCAACACCAGCGGAAGA AAGTCCGTCTGAGTCTGAATCTGCGGCCGCGG EC4 ACTAGTCGGCTCCCCCAGGATGCCATTTGCTCGGATTGGTC SEQ CGCATGGAGCCCCTGCAGTGTATCCTGCGGTGACGGAAGC ID 31 CAAATCAGGACGCGAACTGAGGTTTCTGCTCCGCAACCTGG AACACCAACATGTCCGGACTGCCCCGCGCCCATGGGAAGG ACTTGCGTGGAACAAGGCGGACTTGAAGAAATCCGTGAATG CAGTGCGGGGGTATGTGCTGTTGACGCTGGATGTGGCGTCT GGTCTGGTGGCGGTAGCAGGACTGGATGTCATGCCTTCAG GGAGAACTGCCGCCCTGGTAGATGTATTGATGACGCCTCGC ATGAGAATGGCTACACCTGCGAGTGCCCCACATGGTACTCA CGTGAGGTGACTTCCAAGGCGGAGGAGTCGTGTGTGGAAG GAGTCGAAGTCACGCTGGCTGAGAAATGCGAGAAGGAATTC GGCATCAGCGCGTCCTCCTGCAAATGCGATAACTCTGGTGG CGGTAGTAACGAACCGGTGGCCCTAGCTCAGCTCAGCACAT TCCTCGAGCTCGTCGAGGTGCCATGTAACTCTGTTCATGTTC AGGGGGTGATGACCCCGAATCAAATGGTCAAAGTGACTGGT GCAGGATGGGATAATGGCGTTCTCGAGTTCTATGTCACGAG GCCAACGAAGACAGGCGGGGACACAAGCCGAAGCCACCTT GCGTCGATCATGTGTTATTCCAAGGACATTGACGGCGTGCC GTCAGACAAAGCGGGAAAGTGCTTTTTGAAGAACTTTTCTGG TGAAGACTCGTCGGAAATAGACGAAAAAGAAGTATCTCTACC CATCAAGAGCCACAACGATGCGTTCATGTTCGTTTGTTCTTC AAATGATGGATCCGCACTCCAGTGTGATGTTTTCGCCCTTGA TAACACCAACTCTAGCGACGGGTGGAAAGTGAATACCGTGG ATCTTGACGTCAGCGTTAGTCCGGATTTGGCATTCGGACTCA CTGCAGATGGGGTCAAGGTGAAGAAGTTGTACGCAAGCAGC GGCCTGACAGCGATCAACGACGACCCTTCCTTGGGGTGCAA GGCTCCTCCCCATTCTCCGCCGGCCGGAGAGGAACCGAGT TTGCCGTCGCCTGAAAACAGCGGGTCTGCAACACCAGCGGA AGAAAGTCCGTCTGAGTCTGAATCTGCGGCCGCGG

The chimeric protein EC2 has the amino acid sequence TSRLPQDAICSDWSAWSPCSVSCGDGSQIRTRTEVSAPQPGTPTCPDCPAP MGRTCVEQGGLEEIRECSAGVCAVDAGCGVWSGGGSRTGCHAFRENCSPG RCIDDASHENGYTCECPTGYSREVTSKAEESCVEGVEVTLAEKCEKEFGISAS SCKCDNSGGGSSWNNVARCSYGADSTLGPVKLSAEGPTTMTLVCGKDGVK VPQDNNQYCSGTTLTGCNEKSFKDILPKLTENPWQGNASSDKGATLTIKKEAF PAESKSVIIGCTGGSPEKHHCTVKLEFAGMGSAKSASGR (SEQ ID 28) and its use as recombinant antigen, containing multiple Toxoplasma gondii protein fragments, is covered by the present invention.

The chimeric protein EC3 has the amino acid sequence TSRLAALGGLADQPENHQALAEPVTGVGEAGVSPVNEAGESYSSATSGVQE ATAPGAVLLDAIDAESDKVDNQAEGGERMKKVEEELSLLRRELYDRTDRPGL SGGGSATAATASDDELMSRIRNSDFFDGQAPVDSLRPTNAGVDSKGTDDHLT TSMDKASVESQLPRREPLETEPDEQEEVHFSGGGSNEPVALAQLSTFLELVE VPCNSVHVQGVMTPNQMVKVTGAGWDNGVLEFYVTRPTKTGGDTSRSHLA SIMCYSKDIDGVPSDKAGKCFLKNFSGEDSSEIDEKEVSLPIKSHNDAFMFVCS SNDGSALQCDVFALDNTNSSDGWKVNTVDLGVSVSPDLAFGLTADGVKVKKL YASSGLTAINDDPSLGCKAPPHSPPAGEEPSLPSPENSGSATPAEESPSESES AAA (SEQ ID 30) and its use as recombinant antigen, containing multiple Toxoplasma gondii protein fragments, is covered by the present invention.

The chimeric protein EC4 has the amino acid sequence TSRLPQDAICSDWSAWSPCSVSCGDGSQIRTRTEVSAPQPGTPTCPDCPAP MGRTCVEQGGLEEIRECSAGVCAVDAGCGVWSGGGSRTGCHAFRENCRPG RCIDDASHENGYTCECPTWYSREVTSKAEESCVEGVEVTLAEKCEKEFGISAS SCKCDNSGGGSNEPVALAQLSTFLELVEVPCNSVHVQGVMTPNQMVKVTGA GWDNGVLEFYVTRPTKTGGDTSRSHLASIMCYSKDIDGVPSDKAGKCFLKNF SGEDSSEIDEKEVSLPIKSHNDAFMFVCSSNDGSALQCDVFALDNTNSSDGW KVNTVDLDVSVSPDLAFGLTADGVKVKKLYASSGLTAINDDPSLGCKAPPHSP PAGEEPSLPSPENSGSATPAEESPSESESAAA (SEQ ID 32) and its use as recombinant antigen, containing multiple Toxoplasma gondii protein fragments, is covered by the present invention.

Construction of DNA Vectors Directing the Expression of Chimeric Antigens as Fusion Products with GST in the Cytoplasm of E. coli Cells

DNA fragments encoding for the EC2, EC3 and EC4 chimeric proteins were cloned as fusion products with the protein Glutathione Sulpho Transferase (GST) and expressed as soluble proteins in the cytoplasm of bacterial cells, for the purpose of determining their specificity and selectivity. DNA sequences of EC2, EC3 and EC4 (SEQ ID 27, 29 and 31, respectively) were digested with the restriction enzymes SpeI and NotI. Digested DNA were cloned into vector pGEX-SN (Minenkova et al., International Journal of Cancer, 2003, 106:534-44), which was previously digested with SpeI and NotI endonucleases, to generate fusion products at the carboxy terminus of GST protein. The resulting plasmids were used to transform competent E. coli cells following standard protocols (Sambrook et al., 1989, Molecular Cloning, Cold Spring Harbor Laboratory Press, Cold Spring Harbor).

Biochemical Characterisation of the Recombinant Chimeric Antigens

The recombinant proteins GST-EC2, GST-EC3 and GST-EC4 were expressed in the cytoplasm of transformed E. coli cells and purified by affinity chromatography using Glutathione-Sepharose resin (Amersham Pharmacia Biotech, Sweden), following the manufacturer's instructions. Protein purity and concentration were assessed by SDS-PAGE (Sodium Dodecyl Sulphate-Poly-acrylammide Gel Electrophoresis) analysis and Bradford assay, respectively. The affinity-purified recombinant products were dialyzed against PBS, diluted at a concentration of 1 mg/ml with PBS and stored at −20° C. until use. The yield of purified products were 8 mg/liter of bacterial culture, 5 mg/liter and 4 mg/liter for the chimeric antigens GST-EC2, GST-EC3 and GST-EC4, respectively. The recombinant proteins were subsequently subjected to high-performance liquid chromatography (HPLC) analysis. To this aim a gel filtration was performed using a TSK G4000 SW-XL HPLC-column, by injecting 30 μl of each samples (protein concentration, 2 mg/ml) into the column with a flow rate of 1 ml/min (mobile phase: KH₂PO₄ 0.05 M; NaCl 0.3 M pH 7.0). Results from HPLC analysis demonstrated that the chimeric antigens GST-EC2, GST-EC3 and GST-EC4 were purified as homogeneous products in dimeric forms, with a respective molecular weight of 110.4, 152.6 and 130 kDa (Da, Dalton). Protein aggregates of high molecular weight were absent in all of the purified protein preparations.

Immunoreactivity of the Chimeric Recombinant Antigens with IgG Antibodies from Sera of T. gondii Infected Individuals: IgG Rec-ELISA

The ELISA performance of the GST fusion products was performed by coating Maxisorp-multiwells plates (Nunc) with single antigen fragments or chimeric proteins at a concentration of 5 μg/ml in coating buffer (50 mM NaHCO₃, pH 9.6). After incubation overnight at 4° C. plates were incubated for 1 h at 37° C. with blocking buffer (5% non-fat dry milk, 0.05% Tween-20 in PBS) and subsequently incubated for 1 h at 37° C. with sera from T. gondii seropositive and seronegative individuals, diluted 1:200 in blocking solution. Plates were extensively washed with 0.05% Tween-20 in PBS and anti-human-IgG horse-radish peroxidase-conjugated antibodies (1 mg/ml; Sigma-Aldrich, USA), diluted 1:10000 in blocking solution, was then added to each well. Finally, incubating plates with the chromogenic substrate tetramethylbenzidine (TMB; Sigma-Aldrich, USA) revealed the enzymatic activity. Results were recorded as the difference between the absorbance (Optical Density, OD) at 450 and 620 nm, detected by an automated ELISA reader (Labsystem Multiskan, Finland). For each serum sample the assay was done in triplicate and average values were calculated.

The following Table 3 shows the IgG reactivity of the single antigen fragments and EC2, EC3 and EC4 chimeric antigens, expressed as GST fusion proteins, by using sera from 36 (T1-T36) T. gondii-seropositive and 27 (N1-N27) T. gondii-seronegative humans. The Toxoplasma-specific IgG levels, calculated as International Units (IU) using a commercial assay (VIDAS system, bioMerieux, Marcy-l'Etoile, France) are reported. For every GST-fusion product the cut-off value was determined as the mean plus two times the standard deviation of the absorbency readings obtained from the Toxoplasma IgG negative sera. Normal type, OD<cut-off; bold type, OD>cut-off. The numerical value reported into each cell was calculated as the ratio of the test OD to the cut-off of the corresponding antigen (nd, not determined). Values greater than 1 indicate a positive response.

Table 3 clearly shows that some of the sera which are positive result to be negative when using single antigenic fragments, whereas they result to be positive (correctly) when using the chimeric antigens of the present invention. Please note that the numerical values of IgG concentrations obtained with the standard assay cannot be compared with the others, because they have been calculated by reference to an International Standard, which cannot be used for the other assays.

TABLE 3 Serum IgG levels Recombinant GST-fusion proteins sample (IU/ml) Tx-15.11 Tx-1.11 Tx-1.16 Tx-4.18 Tx-2.a Tx-11.b EC2 EC3 EC4 T1 >300 7.4 3.5 9.8 3.7 18.3 12.5 11.0 10.0 7.6 T2 188 8.7 7.4 2.4 5.1 18.8 4.4 26.0 23.5 20.0 T3 265 7.4 2.8 4.6 2.8 8.4 5.6 20.4 17.9 34.0 T4 4090 23.3 27.8 22.4 29.7 25.1 27.4 33.6 43.3 40.5 T5 >300 12.4 9.7 25.7 2.9 nd nd 30.6 34.4 19.5 T6 35 1.2 0.8 1.0 1.3 0.9 1.4 1.4 5.7 1.7 T7 58 1.0 1.3 0.6 2.3 1.2 1.0 4.6 5.1 1.2 T8 101 2.5 0.9 2.5 2.8 11.2 2.9 17.0 4.8 30.9 T9 88 16.2 5.9 2.4 2.7 6.8 6.7 9.4 27.4 29.9 T10 188 1.7 1.2 4.4 3.1 1.7 1.3 4.4 3.6 3.3 T11 530 28.9 12.7 12.1 31.4 32.1 31.2 32.1 42.6 41.5 T12 89 0.9 nd 4.8 nd 1.2 0.5 1.7 2.4 2.1 T13 1095 8.6 2.3 3.4 5.9 nd nd 31.7 23.4 20.3 T14 248 12.6 4.3 2.1 0.9 nd nd 7.4 14.6 9.2 T15 155 17.5 1.0 2.8 3.4 nd nd 6.7 20.1 13.0 T16 427 4.5 3.9 5.0 2.7 nd nd 17.4 37.9 18.9 T17 236 12.1 4.8 6.1 3.7 nd nd 22.0 33.9 12.7 T18 46 2.1 2.0 2.6 2.2 nd nd 1.9 5.4 1.4 T19 247 2.3 2.5 3.6 0.5 nd nd 3.7 3.9 2.3 T20 100 nd nd nd nd 7.5 1.4 13.1 5.2 14.6 T21 27 2.6 0.6 4.1 1.0 nd nd 3.1 4.8 5.5 T22 >300 28.6 5.1 4.6 19.9 nd nd 14.5 14.8 7.4 T23 92 1.7 1.1 1.3 0.8 0.7 0.8 6.9 4.5 6.4 T24 68 0.5 2.1 2.0 2.6 1.3 1.3 2.5 6.6 1.4 T25 63 2.4 2.4 5.5 6.8 8.8 7.7 13.8 16.6 32.9 T26 299 12.1 10.0 6.5 4.6 nd nd 10.0 13.5 6.0 T27 108 3.4 4.0 5.7 3.0 nd nd 4.8 6.2 3.1 T28 68 1.7 2.7 3.3 2.1 2.0 2.3 6.4 6.9 7.7 T29 >300 1.0 1.1 2.8 1.2 4.0 6.3 9.1 13.7 23.5 T30 114 1.8 2.8 19.3 3.8 3.9 2.0 13.0 13.3 13.6 T31 35 3.2 1.2 3.5 0.9 1.5 1.0 3.6 6.3 5.7 T32 300 4.8 16.0 17.3 20.9 nd nd 31.6 33.1 8.4 T33 123 5.6 2.6 3.3 4.5 20.4 2.8 25.8 21.0 36.6 T34 60 nd nd nd nd 6.2 4.2 9.1 7.7 9.3 T35 155 0.7 2.4 0.5 2.3 1.8 1.1 7.3 10.4 9.5 T36 45 0.3 1.6 1.1 0.8 1.2 0.9 3.2 3.2 3.1 N1 0.4 0.4 0.7 0.7 0.5 0.3 0.4 0.7 0.5 N2 0.7 0.5 0.5 0.4 nd nd 0.4 0.5 0.4 N3 nd nd nd nd nd nd 0.9 0.5 0.6 N4 nd nd nd nd nd nd 0.4 0.5 0.7 N5 nd nd nd nd 0.5 0.7 0.4 0.5 0.5 N6 nd nd nd nd nd nd 0.4 0.5 0.5 N7 0.4 0.4 0.6 0.7 nd nd 1.0 0.5 0.7 N8 0.4 0.5 0.5 0.5 nd nd 0.4 0.5 0.6 N9 0.4 0.6 0.7 0.5 nd nd 0.4 0.5 0.4 N10 nd nd nd nd nd nd 0.3 0.5 0.4 N11 nd nd nd nd nd nd 0.4 0.5 0.5 N12 nd nd nd nd nd nd 0.5 0.7 0.8 N13 nd nd nd nd nd nd 0.4 0.7 0.3 N14 0.7 0.5 0.5 0.5 0.9 0.7 0.5 0.5 0.5 N15 0.4 0.6 0.4 1.0 0.5 0.8 0.4 0.5 0.4 N16 0.8 0.2 0.5 0.6 0.5 0.4 0.5 0.7 0.5 N17 0.8 0.5 0.9 0.4 0.6 0.7 0.4 0.5 0.6 N18 0.3 0.4 0.7 0.5 0.6 0.6 0.4 0.6 0.9 N19 0.4 0.5 0.5 0.8 nd nd 0.5 0.8 0.8 N20 0.6 0.6 0.4 0.6 0.7 0.5 0.8 0.7 1.0 N21 0.7 0.7 0.6 0.5 0.5 0.4 0.4 0.7 0.6 N22 0.4 0.5 0.7 0.5 0.5 0.7 0.4 0.6 0.3 N23 0.5 0.5 0.7 0.5 nd nd 0.4 0.6 0.5 N24 nd nd nd nd 0.5 0.5 0.5 0.7 0.7 N25 nd nd nd nd 0.8 0.5 0.8 0.6 0.5 N26 nd nd nd nd 0.5 0.6 0.4 0.6 0.4 N27 nd nd nd nd 0.4 0.6 0.8 1.0 0.4

The following Table 4 summarizes the results of the ELISA assays based on recombinant proteins, employing serum samples from T. gondii seropositive and seronegative humans. In each column are reported the number and the corresponding percentages of reactive sera. From Table 4 it clearly results that the sensitivity of the assay (see the 2^(nd) column reporting the occurrence of false negatives) is improved when using the chimeric antigens of the invention. This improvement is evident with respect to both the use of single antigenic fragments and the use of a collection or mixtures (Mix-Tx-1.16/Tx-4.18/Tx-2.a, Mix-Tx-1.16/Tx-4.18/Tx-2.a, Mix-Tx-1.16/Tx-2.a/Tx-11.b) of the single different antigenic fragments.

TABLE 4 Sera from T. gondii Sera from T. gondii GST-fusian protein infected subjects uninfected subjects Tx-15.11 28/34 (82.4%) 0/15 Tx-1.11 29/33 (87.9%) 0/15 Tx-1.16 31/34 (91.2%) 0/15 Tx-4.18 27/33 (81.8%) 0/15 Tx-2.a 21/23 (91.3%) 0/14 Tx-11.b 18/23 (78.3%) 0/14 Mix-Tx-1.16/Tx-4.18/Tx-2.a 35/36 (97.2) 0/27 Chimera EC2 36/36 (100%) 0/27 Mix-Tx-15.11/Tx-1.11/ 35/36 (97.2%) 0/27 Tx-11.b Chimera EC3 36/36 (100%) 0/27 Mix-Tx-1.16/Tx-2.a/Tx-11.b 34/36 (94.4%) 0/27 Chimera EC4 36/36 (100%) 0/27 Immunoreactivity of Single Antigenic Domains within the Chimeric Recombinant Antigens with IgG Antibodies of Sera from T. gondii Infected Individuals

To verify that the chimeric antigens retain the immunoreactivity of the single antigen fragments used for their construction, human sera that specifically reacted, in ELISA assays, with single antigen fragments, were adsorbed with different combinations of single antigens and then assayed with the chimeric proteins. To this aim, distinct combinations of the antigen fragments, expressed as GST-fusion products, were coated onto Maxisorb-multiwells plates (Nunc) at a concentration of 10 μg/ml in coating buffer (50 mM NaHCO₃, pH 9.6) and then incubated overnight at 4° C. The plates were extensively washed and subsequently incubated for 30 min. at 37° C. with serum samples (20 μl/well) in blocking solution (5% non-fat dry milk, 0.05% Tween-20 in PBS). The fragment-specific antibody-depleted sera were recovered from each well, added to a new well, incubated for 30 min., and the same procedure was repeated 6 more times. Samples that have been depleted for specific antibodies against a single or multiple antigen fragments were finally analyzed by ELISA assays on the chimeric antigens. For this purpose, the chimeric antigens EC2, EC3 and EC4, as GST-fusion products, were coated overnight at 4° C. onto Maxisorb-multiwells plates at a concentration of 5 μg/ml. The coated plates were blocked and subsequently incubated for 1 h at 37° C. with the antibody-depleted human sera diluted 1:100 in blocking solution. Plates were extensively washed and anti-human-IgG alkaline phosphatase-conjugated antibodies (Sigma-Aldrich, USA), diluted 1:7500 in blocking solution, was then added to each well. Finally, the chromogenic substrate p-nitrophenyl phosphate (Sigma-Aldrich, USA) revealed the enzymatic activity. The results were recorded as the difference between the absorbance at 405 and 620 nm, detected by an automated ELISA reader (Labsystem Multiskan, Finland). For each sample the assay was done in duplicate and average values were calculated.

Biochemical Modification of EC2 and EC3 Chimeric Antigens

To analyze the immunoreactivity of the chimeric antigens EC2 and EC3 with specific anti-Toxoplasma IgM antibodies in patient sera, the recombinant proteins were chemically modified by biotinylation. To this aim, the purified GST-EC2 and GST-EC3, diluted at a concentration of 1 mg/ml in PBS were incubated in the presence of a five-fold molar excess of sulfosuccinimidyl-6-(biotin-amido)hexanoate (Sulfo-NHS-LC-Biotin from Pierce, USA) for 3 hours on ice. The proteins were then dialyzed overnight against PBS to remove excess of non-reacted and hydrolyzed biotin reagents. Levels of biotin incorporation into chimeric antigens was determined by using “EZ Biotin Quantitation Kit” (Pierce, USA), resulting in 1.4 biotin/molecule for GST-EC2 and 1.3 biotin/molecule for GST-EC3. The biotin-labeled products were finally diluted at a concentration of 0.5 mg/ml and stored at −20° C. until use.

Immunoreactivity of the Biotin-Labeled EC2 and EC3 with Toxoplasma-Specific IgM Antibodies: IgM Rec-ELISA

To investigate the immunoreactivity of recombinant antigens with Toxoplasma-specific immunoglobulins M, a double-sandwich immunoassay was employed (IgM Rec-ELISA). Maxisorb plates (Nunc, USA) were coated with anti-human IgM antibodies (Sigma-Aldrich, USA) at a concentration of 10 μg/ml in coating buffer (50 mM NaHCO₃, pH 9.6). Plates were blocked with 3% bovine serum albumin in PBS (blocking solution) for 1 h at 37° C. and subsequently incubated for 1 h at 37° C. with serum samples in blocking solution. Plates were washed and then incubated for 2 h at room temperature with the biotin-labeled GST-fusion proteins, diluted in blocking solution. After being extensively washed the plates were incubated for 1 h at room temperature with horseradish peroxidase-conjugated streptavidin (Pierce, USA) at a concentration of 1 μg/ml in blocking solution. Finally, the enzymatic activity was revealed incubating plates for 30 min. at room temperature with the substrate tetramethylbenzidine (Sigma-Aldrich, USA). Results were recorded as the difference between the absorbance at 450 and 620 nm, detected by an automated ELISA reader (Labsystem Multiskan, Finland). For each sample the assay was done in duplicate and average values were calculated.

Thermal Stability of the Biotin-Labeled Chimeric Antigens

In order to determine the thermal stability of the biotin-labeled GST-EC2 and GST-EC3, recombinant products were diluted at a concentration of 5 μg/ml in the commercial buffer “Stabilzyme” (SurModics, USA) and stored at +4° C. until use. After different interval times (up to 80 days), the immunoreactivity of recombinant proteins in the IgM Rec-ELISA analysis was assessed and results obtained analyzing the corresponding products maintained frozen at −20° C. were compared. The IgM Rec-ELISA was performed as described above, using the biotin-labeled GST-EC2 and GST-EC3 antigens at a final concentration of 500 ng/ml in blocking solution (3% BSA in PBS) and human sera diluted 1:100 in blocking solution. For each sample the assay was done in duplicate and average values were calculated. The ID50, calculated as day-limit when the 50% of toxoplasma-specific IgM-immunoreactivity was measured, were 189 days and 97 days for GST-EC2 and GST-EC3, respectively. These findings clearly indicate that the chimeric antigens of the invention are stable in diluted solutions for a long time, which a fundamental requisite for the commercial usefulness of a recombinant product.

Expanded Evaluation of IgM Rec-ELISA

The biotin-labeled GST-EC2 and GST-EC3 chimeric antigens were assayed with IgM antibodies in sera from T. gondii infected individuals and the results of the IgM Rec-ELISAs were compared with those obtained with commercial assays employing lysed, whole-cell Toxoplasma antigen (VIDAS system from bioMerieux, France; ETI-TOXOK-M Reverse-PLUS from DiaSorin, Italy). To this aim, serum samples from women who acquired primary toxoplasmosis during gestation and referred for post-natal follow-up at the Center for Perinatal Infection of Campania Region, Italy, were assayed. The bioMerieux VIDAS Toxo IgG and IgM assays were used to select three groups of serum samples for the Toxoplasma IgM Rec-ELISA performance evaluation. Group A (n=22) was composed of samples negative for T. gondii-specific IgM and IgG antibody as measured by the VIDAS Toxo IgM and IgG assays. Group B (n=18) was composed of samples with a serological profile consistent with a chronic infection (presence of T. gondii-specific IgG antibody and absence of T. gondii-specific IgM as measured by the VIDAS Toxo IgM and IgG assays, respectively). Group C (n=50) was composed of samples with a serological profile consistent with an acute infection (presence of T. gondii-specific IgM and IgG antibodies as measured by the VIDAS Toxo IgM and IgG assays). IgM Rec ELISA was performed as described above and for each serum sample the assay was done in duplicate and average values were calculated.

The following Table 5 shows the IgM reactivity of the biotin-labeled GST-EC2 and GST-EC3 chimeric antigens, compared to the results obtained with commercial assays (VIDAS and ETI-TOXO-K), by using sera from group A (A1-A22), group B (B1-B18) and group C (C1-C50). The Toxoplasma-specific IgG levels, calculated as International Units (IU) are also reported. For each biotin-labeled GST-fusion product the cut-off was determined as the mean plus two times the standard deviation of the absorbency readings obtained from the T. gondii-specific IgM negative sera (groups A and B, n=40). Cut-off values for VIDAS IgM, ETI-TOXOK-M Reverse-PLUS, GST-EC2 and GST-EC3 were 0.650, 0.500, 0.343 and 0.378, respectively. Values typed in bold indicate a positive response.

TABLE 5 Toxo-IgG VIDAS ETI-TOXOK-M IgM Rec-ELISA Serum (UI/ml) IgM Reverse-PLUS GST-EC2 GST-EC3 A1 0 0.05 0.397 0.268 0.256 A2 4 0.22 0.317 0.263 0.270 A3 0 0.18 0.252 0.264 0.237 A4 0 0.05 0.375 0.324 0.241 A5 2 0.17 0.272 0.298 0.222 A6 0 0.03 0.288 0.270 0.234 A7 0 0.19 0.210 0.215 0.379 A8 0 0.10 0.108 0.203 0.296 A9 0 0.06 0.324 0.314 0.291 A10 0 0.09 0.339 0.325 0.286 A11 0 0.05 0.193 0.223 0.271 A12 2 0.08 0.134 0.268 0.378 A13 4 0.12 0.115 0.309 0.335 A14 0 0.23 0.115 0.221 0.286 A15 0 0.06 0.230 0.281 0.374 A16 2 0.08 0.132 0.317 0.269 A17 1 0.18 0.123 0.277 0.281 A18 0 0.35 0.097 0.316 0.279 A19 0 0.28 0.346 0.274 0.272 A20 0 0.09 0.054 0.259 0.132 A21 0 0.61 0.206 0.24 0.312 A22 0 0.06 0.127 0.238 0.233 B1 24 0.06 0.239 0.255 0.189 B2 10 0.09 0.076 0.283 0.304 B3 44 0.14 0.124 0.265 0.261 B4 44 0.28 0.195 0.298 0.216 B5 25 0.1 0.131 0.273 0.185 B6 57 0.12 0.164 0.296 0.293 B7 12 0.22 0.185 0.257 0.194 B8 58 0.12 0.148 0.255 0.248 B9 56 0.4 0.174 0.232 0.268 B10 19 0.09 0.068 0.290 0.194 B11 56 0.16 0.136 0.179 0.221 B12 45 0.12 0.139 0.235 0.181 B13 87 0.12 0.096 0.207 0.218 B14 27 0.15 0.144 0.196 0.174 B15 33 0.46 0.242 0.285 0.378 B16 13 0.04 0.064 0.161 0.177 B17 67 0.13 0.111 0.177 0.213 B18 53 0.27 0.170 0.238 0.165 C1 28 5.37 1.04 0.350 0.548 C2 255 3.86 1.49 0.546 0.498 C3 78 3.01 1.38 0.471 0.867 C4 1358 2.28 1.17 0.464 0.453 C5 178 2.31 1.27 0.598 0.406 C6 155 2.00 0.97 0.993 0.720 C7 109 3.20 1.76 0.794 0.642 C8 99 3.16 1.78 0.572 0.389 C9 103 2.28 1.34 1.056 1.222 C10 85 2.22 1.44 0.930 0.704 C11 70 1.01 0.79 0.416 0.376 C12 26 1.34 0.93 0.392 0.461 C13 36 1.22 0.86 0.532 0.499 C14 156 0.99 0.58 0.534 0.833 C15 204 0.93 0.90 0.810 0.710 C16 133 1.13 0.85 0.465 0.322 C17 183 1.14 0.82 0.320 0.327 C18 242 0.90 0.71 0.497 0.500 C19 80 1.00 0.79 0.444 0.706 C20 258 1.40 0.88 2.678 0.484 C21 278 1.69 1.06 0.703 0.509 C22 246 1.25 0.76 1.094 0.780 C23 59 1.23 0.71 0.495 1.499 C24 38 0.78 0.87 0.584 0.455 C25 130 0.76 0.92 0.562 0.545 C26 262 0.84 0.65 0.649 0.551 C27 168 0.96 0.85 1.439 0.938 C28 126 0.78 0.80 2.475 1.160 C29 197 1.38 0.61 0.544 0.358 C30 127 0.86 0.52 0.847 0.531 C31 72 1.28 0.93 1.756 0.891 C32 130 0.77 0.71 0.505 0.381 C33 439 1.00 0.66 0.834 0.464 C34 83 0.66 1.32 1.162 0.989 C35 178 0.89 0.87 0.694 0.487 C36 560 0.86 0.69 0.817 0.628 C37 223 0.96 0.73 0.531 0.819 C38 242 0.98 0.41 0.379 0.318 C39 118 1.16 0.84 0.420 0.380 C40 232 1.39 1.01 0.490 0.467 C41 213 1.03 1.05 0.750 0.822 C42 243 1.06 0.97 0.534 0.502 C43 154 0.75 0.73 0.455 0.337 C44 35 1.90 1.51 0.383 1.008 C45 667 0.85 1.01 0.366 0.285 C46 275 0.95 0.99 0.411 0.544 C47 157 1.93 1.36 0.382 0.464 C48 1037 1.08 0.51 0.385 0.301 C49 92 1.31 0.68 0.537 0.427 C50 255 0.69 0.69 0.354 0.801

The following Table 6 shows the performance characteristics of the commercial assays (VIDAS IgM and ETI-TOXOK-M Reverse PLUS), compared to the results obtained with the biotin-labeled EC2 and EC3 chimeric antigens (IgM Rec-ELISA). From Table 6 it clearly results that both specificity and positive predictive values of the assays (see the 3^(rd) column reporting the occurrence of false positives) reached the maximum (100%) when using the chimeric antigens of the invention. With regard to sensitivity and agreement, both the commercial test ETI-TOXOK-M employing lysed, whole-cell Toxoplasma antigen and the IgM rec-ELISA with the chimeric antigen EC2 display identical performance characteristics, with both values very close to 100%.

TABLE 6 Diagnostic Sensitivity Specificity Agreement PPV* NPV* test (%) (%) (%) (%) (%) VIDAS IgM 100 100 100 100 100 ETI-TOXOK-M 98.0 100 98.9 100 97.6 EC2-IgM 98.0 100 98.9 100 97.6 Rec-ELISA EC3-IgM 84.0 100 91.1 100 83.3 Rec-ELISA *PPV, positive predictive value; NPV, negative predictive value.

Finally, the immunoreactivity of the biotin-labeled GST-EC2 and GST-EC3 antigens with IgM antibodies in sera from infants with congenital toxoplasmosis was investigated. In a retrospective study, sera from 30 infants of mothers with primary T. gondii infection during pregnancy were analyzed. Twenty infants had congenital toxoplasmosis and ten was uninfected, as demonstrated by the persistence or disappearance of Toxoplasma-specific IgG antibodies after 12 months of age, respectively. Within the infected patient cohort, the gestational age at the time of maternal infection was the second trimester in 6 mothers and the third trimester in 14 mothers. 30 serum samples from infected and uninfected infants were analyzed by IgM Rec-ELISA, and results obtained with commercial assays employing the whole-cell Toxoplasma antigen (ELFA-IgM and ETI-TOXOK-M Reverse PLUS) were compared. Specific levels of anti-Toxoplasma IgG ranged from 28 to 1147 IU/ml for sera from infected infants and from 19 to 170 IU/ml for sera from uninfected subjects. For every GST-fusion product the cut-off value was determined as the mean plus 2SD of the absorbency readings obtained with sera from uninfected infants. In Table 7 are summarized the results of the IgM Rec-ELISAs with individual sera from infected infants. Overall, the number of IgM-reactive sera ranged from 70% (14/20) to 50% (10/20) using the GST-EC2 and GST-EC3 antigens, respectively. In contrast, only 7 out of 20 infected infants (35%) had positive results when ELFA-IgM or ETI-TOXOK-M assays were employed. Among uninfected infants, none of the sera recognized GST-EC2 and GST-EC3 antigens in the IgM Rec-ELISA or resulted to be positive using commercial assays.

In conclusion, these results demonstrate that the use of recombinant chimeric antigens is effective in distinguishing T. gondii-infected from uninfected individuals, having comparable or even better assay performance with respect of using the whole-cell tachyzoite antigen, and could provide the basis for standardized commercial immunoassays for toxoplasmosis serodiagnosis.

TABLE 7 Toxoplasma-specific IgM reactivity of serum samples from 30 infants born to mothers with primary T. gondii infection acquired during pregnancy^(a) Time IgM Rec-ELISA after IgG ELFA- ETI- cutoff^(d) Patient birth levels IgM ToxoM GST- GST- no. (wk) Onset^(b) (IU/ml) cutoff^(c) cutoff^(c) EC2 EC3 T1 1 B 169 6.41 2.66 2.479 0.542 T2 2 B 988 0.73 0.15 0.360 0.270 T3 2 Sub 300 0.09 0.13 0.212 0.209 T4 3 Sub 57 0.05 0.23 0.206 0.211 T5 3 Sub 124 0.13 1.62 0.641 1.103 T6 4 Sub 218 0.04 0.09 0.452 0.269 T7 4 S 157 2.61 1.62 1.522 0.225 T8 4 S 172 3.98 1.50 1.804 0.353 T9 5 S 1147 0.07 0.10 0.519 0.206 T10 5 B 47 0.11 0.12 0.272 0.276 T11 6 Sub 28 0.10 0.18 2.617 0.731 T12 6 Sub 136 0.07 0.11 0.314 0.216 T13 7 S 209 0.88 0.47 0.683 0.217 T14 8 Sub 43 0.06 0.07 0.196 0.213 T15 8 B 160 0.82 0.08 0.206 0.219 T16 8 B 64 0.02 0.40 0.231 0.228 T17 8 Sub 145 0.31 0.57 0.985 0.314 T18 9 Sub 300 6.37 1.30 0.548 0.315 T19 12 Sub 196 0.05 0.17 0.463 0.268 T20 12 Sub 75 0.05 0.07 0.237 0.222 N1 5 90 0.38 0.06 0.237 0.218 N2 5 170 0.06 0.07 0.204 0.200 N3 5 66 0.23 0.09 0.238 0.235 N4 3 44 0.07 0.12 0.176 0.209 N5 9 41 0.05 0.05 0.209 0.193 N6 5 66 0.07 0.06 0.194 0.196 N7 8 13 0.09 0.06 0.193 0.201 N8 9 13 0.14 0.06 0.208 0.231 N9 6 19 0.28 0.07 0.184 0.215 N10 5 20 0.13 0.05 0.189 0.240 Notes to Table 7 ^(a)Serum samples from T. gondii infected (T1-T20) or uninfected children (N1-N10) were analyzed by IgM Rec-ELISAs with GST-EC2 and GST-EC3 antigens or by commercial assays (ELFA-IgM and ETI-TOXO-M). ^(b)Severity of clinical onset: S. severe; B. benign; Sub. subclinical. ^(c)Cutoff values for the ELFA-IgM and ETI-TOXO-M assays were 0.65 and 0.41 as indicated by manufacturers. respectively. Bold type. values > cutoff. ^(d)Cutoff values for the IgM Rec-ELISA using GST-EC2 and GST-EC3 antigens were 0.25 and 0.26. respectively. Bold type. values > cutoff.

The paper copy of the sequence listing submitted herewith and the corresponding computer readable form are both incorporated herein by reference in their entirety. 

1. A chimeric recombinant antigen containing a fusion of at least three different antigenic fragments of Toxoplasma gondii polypeptides, wherein said antigenic fragments are B-cell epitopes, which bind to Toxoplasma gondii-specific antibodies and wherein the chimeric recombinant antigen has an amino acid sequence selected from the group consisting of: SEQ ID NO: 28, SEQ ID NO: 30 and SEQ ID NO:
 32. 2. The chimeric recombinant antigen of claim 1, wherein the Toxoplasma gondii-specific antibodies are extracted from sera of subjects who have been infected by Toxoplasma gondii.
 3. A chimeric recombinant antigen encoded by a nucleotide sequence that hybridizes under highly stringent hybridization conditions with a complement of a nucleotide sequence coding for the chimeric recombinant antigen according to claim
 1. 4. The chimeric recombinant antigen according to claim 1, obtained by a process comprising culturing a host cell transformed with a vector comprising a nucleotide sequence that codes for the chimeric recombinant antigen of claim 1 or that hybridizes under highly stringent hybridization conditions with a complement of a nucleotide sequence coding for the chimeric recombinant antigen of claim 1, and isolating the chimeric recombinant antigen from the cultured host cells.
 5. A kit for the diagnosis of Toxoplasma gondii infection, containing at least one chimeric recombinant antigen according to claim
 1. 6. The kit of claim 5, wherein the kit is for the diagnosis of an acute and/or chronic Toxoplasma gondii infection. 